Separating out defined phases of a useful mineral, which in particular are present in the ore with a very fine distribution, from a ground ore always represents a technical problem. This finely distributed presence of useful phases in an ore arises particularly in the case of rare earth phases, but also for other conventional metallic phases, such as copper minerals. Because this separation problem arises more frequently in the case of rare earth elements or rare earth compounds in mineral rock, the description herein more particularly focuses on the extraction of rare earths. However, the method described below can basically be applied to numerous extraction processes for other metals.
The rare earths occur naturally in various minerals, always in an oxidized form, for example as carbonates or phosphates. Although there are numerous minerals, 95% of the world's rare earth resources consist of the three minerals bastnasite, monazite and xenotime. It is characteristic of rare earth minerals that they contain the entire spectrum of rare earth elements. Due to this association, and the great similarity of the rare earth elements in their chemical behavior, the requirements to be met by the process for the separation the individual substances are very demanding. In the case of the rare earth minerals, one characteristic feature which is always technically challenging consists in the fact that in the ore they are generally very finely interspersed, as a result of which the beneficiation process must in addition meet highly demanding requirements. Thus the ore must on the one hand be adequately crushed in order to achieve a sufficient level of exposure of the useful materials. On the other hand, very fine grain sizes often make more difficult the extraction of the useful materials during the production of a concentrate (flotation). In addition to this there is the fact that a large area is required for the quantities of waste material which arise (the flow of waste material, or gangue, referred to below as tailings). A further property of the rare earths is that they are frequently interspersed with such radioactive contaminant materials as thorium and uranium. These are also released during the beneficiation, so that there are also environmental risks. Due to these ecological and economic problems, many deposits of rare earth minerals are nowadays not mined.
In the beneficiation of bastnasite, a typical ore containing rare earth minerals, after the ore has been crushed the broken pieces are ground down to a size suitable for flotation, of less than 150 micrometer. This process involves substantial energy costs. In general, the target grain size for the grinding is determined according to the exposed grain size of the rare earth mineral. This is heavily dependent on the ore type and the deposit concerned. The term exposed grain size is to be understood here as the size of grain in which the individual mineral phases are present as individual grains. Basically, an exposure of 100% should be the aim, in reality it may be that exposures of 50%-70% are realistic, depending on the deposit. If the crushing produces pieces smaller than the exposed grain size, that is the grain size at which the individual mineral phases are present as separate pieces, this is overgrinding of the particles and results in the formation of a high proportion of fine particles. These can often not be extracted by the subsequent flotation, which is used to separate the useful material and the valueless material (gangue, tailings), or can even have a negatively detrimental effect on the process. On the other hand, if the exposed grain size is exceeded, the mineral will not be present in a completely free state, so that the interaction between the surface of the mineral and the chemical agents is reduced or prevented. As a result, the useful material which is to be extracted cannot adhere adequately to the rising gas bubbles during the flotation and thus become enriched in the foam zone of the upper surface of the liquid.
Apart from the efficiency of the extraction, the yield (recovery) from the flotation has a decisive influence on the efficiency of the overall process for the extraction of rare earths. The higher is the yield of rare earths, and hence the enrichment of rare earths in the concentrate, the lower is the loss of useful material in the process. Presently, it is possible to achieve yield levels for rare earths of 65%-70%. But it also follows that 30%-35% of the rare earth materials which are contained in the initial ore are not floated off, and get lost in the tailings. One reason for this is the poor buoyancy of fine particles of material, in particular particles with a grain size of less than 20 micrometers are affected by this. The main reason for this is the low collision efficiency between small particles and gas bubbles. In addition, small particle sizes require a large bubble surface area to bind on the particles of useful material, which with conventional flotation can only be achieved with a high proportion of very small gas bubbles. However, these are in turn not suitable for transporting the larger particles of useful material into the foam layer and in addition, in a conventional flotation process (stirred or mechanical cells; columnar cell), can only be produced at substantial energy cost.
In order to provide a remedy for this, two approaches are applied in principle. One consists in increasing the size of the particles of useful material, or in reducing the size of the gas bubbles. For the purpose of increasing the particle sizes, use is made of selective flocculation, coagulation and hydrophobic aggregation of the particles. These methods require additives such as polymers or electrolytes, and are already being used industrially. By comparison with electrolytes, the advantage of added polymers is their high selectivity, they bind solely with the particles of useful material, and not with the valueless material. However, there are frequently inclusions of gangue (tailings) in the interstices in the aggregates which are formed. Reduction in the size of the gas bubbles is the approach used, for example, in dissolved gas flotation, in electro-flotation and in turbulent micro-flotation. Because the gas bubbles are small, low speeds of upward movement are achieved, so that the small particles can remain attached during the upward movement. However, this results in long residence times for the useful material in the flotation cell. Apart from this, the low speeds of upward movement can have a negative effect on the selectivity.